When did North America become a home where the ancestors of buffalo roamed? Between 195,000 and 135,000 years ago, according to a study published Monday that reports on the oldest fossil and genomic evidence of bison on the continent.

Scientists have set out to chronicle when the massive, furry beasts first crossed into North America from Asia via the Bering land bridge because that event was the beginning of a striking change in the ecology of the continent. They refer to it as an invasion because once the bison arrived, they thrived everywhere and began competing with the horses and mammoths that had grazed the Great Plains for millions of years.

“The only other invasion of North America that has had such an ecological and environmental impact has been us,” said Beth Shapiro, an evolutionary molecular biologist at the University of California, Santa Cruz, and a co-author of the paper, which appeared in the Proceedings of the National Academy of Sciences.

In the millenniums that followed, the ancient bison evolved into the iconic American buffalo we know today.

Dr. Shapiro worked with Duane Froese, an earth scientist from the University of Alberta in Canada and the lead author of the paper, to construct the chronology of the bison colonization.

One key to narrowing down when the bison arrived came from a serendipitous find made by Dr. Froese’s colleague Alberto Reyes, a geologist from the University of Alberta. In 2006 while studying ancient volcanic ash buried in the northern Yukon in northwestern Canada, Dr. Reyes stumbled across a bone jutting from the frozen mud near the ash layer.

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A 130,000-year-old fossil bone from the foot of a bison found in frozen mud in the Yukon, shown next to an excavation tool.CreditAlberto Reyes

He passed it off to Grant D. Zazula, the paleontologist on the site, and returned to his lab to write a paper on the history of permafrost. It was not until several years later while he was working on the paper that something jogged his memory.

“This light bulb went off as I was going through my old field notes,” Dr. Reyes said. “‘Oh my God, I forgot about this bone that I gave to that paleontologist. That bone could be pretty important.’”

It was.

The fossil was of a bone from the front foot of a steppe bison, an extinct species that came from Siberia, believed to be 130,000 years old. That would make it the oldest bison fossil found on the continent. Dr. Reyes determined its age by using the ancient volcanic ash layer near where it was found, which has a known age, as a reference.

For corroboration, the team performed a genetic analysis of the bone. In addition to sequencing the mitochondrial DNA of the bone, the team also sequenced the mitochondrial genome of a 120,000-year-old fossil belonging to an extinct giant longhorn bison that was found in Snowmass Village, Colo., a ski resort town.

Thought to be the largest bison to have ever lived, the giant longhorn was about two tons and more than eight feet tall at the shoulders. It lived in what is now the southern part of Canada and in the contiguous United States.

After sequencing both beasts’ DNAs, the team compared them with the genomes of about 40 other bison fossils from Siberia and North America. It found that the steppe and longhorn bison specimens were genetically very similar and that both were descendants from the first bison to come to North America, even though they looked very different and lived far apart.

The team’s genetic analysis suggests that after bison ancestors first invaded from Asia more than 130,000 years ago, when sea levels are thought to have been much lower, they dispersed throughout the continent within about 20,000 years, much quicker than previously thought. During that time their looks and body sizes changed as they adapted to their specific environments, rapidly giving rise to creatures like the giant longhorn.

“This revises everything we thought about how bison came into North America and evolved after that,” Dr. Shapiro said.

Hendrik Poinar, an evolutionary biologist from McMaster University in Ontario, who was not involved in the study, was impressed with how the team integrated paleontological, geological and genetic information into “a single narrative that is quite convincing.”

He added that “until more fossils come along, this will stand as the best estimate of the timing of bison into North America.”

A version of this article appears in print on , on Page D2 of the New York edition with the headline: Early Immigrants: Bison Quickly Made Homes on the Range. Order Reprints | Today’s Paper | Subscribe